1. Field of the Invention
The present invention relates to a synchronization circuit and a synchronization method that detect an incoming packet transmitted from a communication partner by using the preamble of the packet to perform synchronization, to a wireless communication apparatus and a wireless communication method that receive a packet on the basis of the result of synchronization using the preamble of the packet, and to a computer program. More particularly, the present invention relates to a synchronization circuit, a synchronization method, a wireless communication apparatus, a wireless communication method, and a computer program that estimate a frequency offset, timing, and signal-to-noise ratio (SNR) after a packet is detected from a received signal.
2. Description of the Related Art
Wireless networks draw attention as systems that are free from wiring in wired communication methods in related art. Typical standards concerning the wireless networks include the Institute of Electrical and Electronics Engineers (IEEE) 802.11 and IEEE 802.15. For example, an Orthogonal Frequency Division Multiplexing (OFDM) modulation method, which a multi-carrier method, is adopted in IEEE 802.11a/g as a standard for a wireless local area network (LAN).
Although the modulation method capable of achieving a communication speed up to 54 megabits per second (Mbps) is supported in the IEEE 802.11a/g, a next-generation wireless LAN standard capable of realizing a higher bit rate is demanded. IEEE 802.11n resulting from expansion of the IEEE 802.11 adopts an OFDM-Multi-Input Multi-Output (MIMO) communication method in which multiple antennas are used to perform beamforming in accordance with the channel characteristics.
In wireless communication, an preamble including given repetitive sequences is normally added to the beginning of a packet and a receiver uses the preamble to perform synchronization processing. Specifically, after detection of a preamble allows the packet to be detected, confirmation of reception timing and normalization of the power of a received signal (setting of an automatic gain control (AGC) gain) are precisely performed. After, for example, a frequency offset, an SNR, and a channel are estimated by using a part in the preamble subsequent to the part where the packet has been detected to remove the effects of the frequency offset, the SNR, and the channel, a data symbol is demodulated.
For example, a demodulation timing generation circuit is proposed, which performs AGC and frequency offset correction by using a training signal burst for synchronization added to the beginning of a packet and sets a detection window period for detection of cross correlation to detect a peak of the cross correlation, thereby setting an optimal Fast Fourier Transform (FFT) window, regardless of the status of the transmission path (for example, refer to Japanese Unexamined Patent Application Publication No. 2003-69546).
FIG. 13 shows an example of the frame format in IEEE 802.11a/g. FIG. 14 shows an example of the preamble structure defined in IEEE 802.11a/g.
As shown in FIG. 14, a short training field (STF) of 8.0 μs and a long training field (LTF) of 8.0 μs are added to the beginning of a preamble. In the STF, short preambles t1 to t10, which form a short training sequence (STS), are sequentially transmitted in a burst manner. In the LTF, long preambles T1 to T2, which form a long training sequence (LTS), are sequentially transmitted after a guard interval G12 of 1.6 μs.
Normally, a receiver calculates self correlation between the repetitive STS symbols included in the STF and determines that a packet is detected if (the square of) the absolute value of the self correlation value exceeds a predetermined threshold value. The self correlation is calculated, for example, by accumulating the result of complex conjugate multiplication of each received signal by a signal that is received one repetition period before the received signal or by calculating the moving average of the result of the complex conjugate multiplication.
In a common receiver, after four 0.8-microsecond STS symbols are used to set the AGC gain and correct the DC offset, the remaining six STS symbols are used to estimate and correct the frequency offset, detect a packet, and perform coarse timing detection. For example, upon detection of a packet, the remaining preamble field is used to perform timing detection, measurement of the frequency offset, digital gain control, and so on (for example, refer to Japanese Unexamined Patent Application Publication No. 2004-221940, paragraphs 0158 to 0164 and FIG. 19).
As shown in FIG. 13, a Signal part follows the preamble. Control information necessary for decoding an information part (Data part) of the packet is stored in the Signal part. The control information necessary for decoding of the packet is called a Physical Layer Convergence Protocol (PLCP) header. The PLCP header includes a Rate field indicating the transmission speed of the following information part, a Length field indicating the length of the information part, a parity bit, a tail bit of the encoder, and so on.
The receiver analyzes the Signal part to decode the following information part on the basis of the result of decoding of the Rate field and Length field. If the receiver detects a parity error as the result of parity check using the value described in the parity field, the receiver recognizes the packet error to discard the received signal and starts to search for a packet again.
However, when the receiver erroneously detects a packet in response to a signal other than a desired preamble or a noise, there is a problem in that unnecessary decoding is started to prevent detection of a packet until a parity error is recognized. Since it is not possible to detect any desired incoming packet before a parity error is recognized, the communication capacity can be reduced. In addition, if the parity check fails and no parity error is detected, a packet is not possibly detected for a longer time. It is relatively likely to cause a state in which detection of a parity error is suppressed due to any bit error.
The receiver can change the threshold value to be compared with the self correlation value of the STF to easily adjust the sensitivity of the packet detection. Accordingly, the sensitivity of the packet detection may be set so as not to respond to a signal other than a desired preamble or a noise.
However, a decrease in the threshold value used in the self correlation to improve the sensitivity of the packet detection causes the receiver to sensitively respond to, for example, a noise to unnecessarily detect a packet. The unnecessary detection of a packet can unnecessarily suppress the transmission operation in a media access control (MAC) layer to restrict the communication capacity. Also if the communication apparatus is at the reception side of the data frame, the unnecessary detection of a packet restricts the communication capacity because it is necessary to transmit an Acknowledge (ACK) packet.
Conversely, an increase in the threshold value used in the self correlation prevents detection of a packet if the SNR is low to reduce the communication capacity or disable the communication. Furthermore, the time when the AGC gain is fixed upon incoming of a packet is delayed. This can affect the frequency offset and the channel estimation that will be performed later to cause a decoding error.
Consequently, with the method of setting the sensitivity of the packet detection, it is not possible to sufficiently resolve the above problem that the erroneous detection of a packet causes unnecessary processing to be continued to suppress detection of any desired incoming packet during the processing.